Temporal and spatial distribution of niclosamide in the water column and sediment were evaluated after the application of granular Bayluscide in six lentic Sea Lamprey (Petromyzon marinus) larval assessment plots. Water and sediment were collected 0.25, 1, 3, 5, and 7 hours after application and were analyzed for niclosamide, the active ingredient in granular Bayluscide. Water samples were collected from five heights in the water column (1 cm, 13 cm, 26 cm, ½ water column, and water surface) at five locations inside and four locations 10 m outside of each assessment plot. Sediment was collected from 18 locations within each plot. Niclosamide water concentrations inside and outside of the plots did not vary by depth but did vary between plots and by time. Niclosamide water concentrations also varied by sampler location outside of the plots. Following granular Bayluscide applications the mean niclosamide concentration in water for all levels, within the plots, decreased from 0.12 mg∙L-1 (SD = 0.12 mg∙L-1) at 15 minutes to 0.061 mg∙L-1 (SD = 0.040 mg∙L-1) at hour 1. The mean niclosamide concentration in the top 4 cm of sediment was 2.9 mg∙kg-1 (SD = 2.4 mg∙kg-1) 15 minutes after application and was 1.3 mg∙kg-1 (SD = 1.8 mg∙kg-1) at hour 7. Concentrations in the sediment ranged from 0.000 to 30.730 mg∙kg-1 and varied between the six plots. Niclosamide concentrations measured in sediment samples were more than 1 order of magnitude greater than in the water and varied spatially by over 4 orders of magnitude. The datasets included are as follows: Niclosamide Sediment Concentrations Dataset (NicSed) Niclosamide Water Column Concentrations Dataset (NicWaterColumn) Niclosamide Water Concentrations Outside Plot Dataset (NicWaterOutside) Plot pH, depth and temperature dataset (PlotData) Plot Sediment pH before and after treatment (SedpHPlotTrt) Plot Sediment Temp before and after treatment (SedTempPlotTrt)